The present invention relates to the production of magnetic powder consisting mainly of iron, particularly to the production of magnetic powder materials which have a high coercive force and saturation flux density desirable for preparing magnetic recording media which are capable of recording signals at a high density.
Hitherto, various magnetic powder materials have been proposed for use in preparing magnetic recording media--for example, .gamma.-Fe.sub.2 O.sub.3, Co doped Fe-.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co doped Fe.sub.3 O.sub.4, Fe.sub.3 O.sub.4 -.gamma.-Fe.sub.2 O, CrO.sub.2 etc. Nowadays, though those skilled in the art still aim at development of recording media which are capable of recording signals at a higher density per unit volume of magnetic material used, it has been found that the conventional materials show a serious defect in the recording of signals of relatively short wavelength. That is their magnetic characteristics such as coercive force (Hc) and flux density (.sigma.) are insufficient to achieve a high density recording. Therefore, many efforts have been made to find a magnetic material which is suitable for such application. Ferromagnetic metal and alloy materials are thought to be most feasible. It is known that while .gamma.-Fe.sub.2 O.sub.3 has usually a saturation flux density on the order of 5,000 gauses, metal materials such as metallic Fe and Fe-Co alloys have a saturation flux density as high as about 20,000 to about 25,000 gauses, four or more times greater than the former. Therefore, if complications that might arise in actual practice are ignored, the metal materials should, theoretically, have about four times the reproducing power obtained with the conventional materials and would enable the production of a recording medium to be used for high density recording.
Some of the various prior art processes proposed for producing ferromagnetic metal or alloy powder materials are listed below:
(1) Process for producing a ferromagnetic metal or alloy by thermal decomposition of a metal salt or salts, typically an oxalic acid salt followed by reduction of the decomposition product with a reducing gas: as described in, for example, Japanese Patent Publications Nos. 11412/61, 22230/61, 8027/65, 14818/66, 24032/67, 22394/68, 38417/72 and 29280/73; Japanese Patent Public Disclosures (KOHKAI) Nos. 38523/72, 22346/73 and 22994/73; U.S. Pat. Nos. 3,186,829 and 3,190,748; Japanese Patent Publication No. 4286/72 (employing phthalate salts).
(2) Process comprising a step of reducing an iron compound for example, selected from iron oxyhydroxide, metal doped-iron oxyhydroxides (e.g. Co doped-oxyhydroxide), iron oxides and ferrite oxides; as described in, for example, Japanese Patent Publication Nos. 3862/60, 11520/62, 20939/64, 29706/72, 30477/72, 39477/72, 24952/73, 7313/74 and 5608/76; Japanese Patent Public Disclosures (KOHKAI) Nos. 5057/71, 7153/71, 79153/73, 82393/73 and 135867/74; U.S. Pat. Nos. 3,598,568 (Klomp et al), 3,607,220 (Vander Giessen et al) and 3,702,270 (Kawasaki et al); U.K. Pat. No. 640,438.
(3) Production of a ferromagnetic metal or alloy by evaporation of a corresponding metal component or components in an inert gas atmosphere: as described in, for example, Japanese Patent Publications Nos. 27718/72, 964/73 and 42780/73; Japanese Patent Public Disclosures (KOHKAI) Nos. 25662/73, 25663/73, 25664/73, 25665/73, 31166/73, 55400/73, 80192/73 and 52134/74.
(4) Production of magnetic metal powder by decomposition of metal carbonyl compounds: as described in, for example, Japanese Patent Publication No. 16868/70, U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and 3,228,882.
(5) Production of magnetic metal powder by electrolysis of an appropriate metal salt using a mercury cathode followed by removal of mercury entrained in the electrolytic product: as described in, for example, Japanese Patent Publications Nos. 15525/64, 8123/65 and 6007/72 and U.S. Pat. Nos. 3,156,650 and 3,262,812.
(6) Wet reducing of a salt of ferromagnetic metal in solution with a reducing agent such as sodium hypophosphite or sodium borohydride: as described in, for example, Japanese Patent Publication Nos. 20520/63, 26555/63, 20116/68, 9869/70, 7820/72, 16052/72, 41718/72 and 41719/72; Japanese Patent Public Disclosure (KOHKAI) Nos. 1353/72, 42252/72, 42253/72, 7585/73, 25896/73, 44194/73, 79754/73, 82396/73, 28999/73 and 1998/73; and U.S. Pat. Nos. 3,607,218, 3,494,760, 3,535,104, 3,661,556, 3,663,318, 3,669,643, and 3,672,867.
(7) Others, for example a method comprising a step of passing a current impulse through a wire of a magnetic metal placed in vacuo sufficient to cause the wire to be broken into fine powder by discharge-detonation as described in Japanese Patent Publication No. 33857/72.
However, in practice none of these known methods have proved to be commercially feasible.
For example, though the above method (1) which comprises decomposition of a metal salt such as an oxalate has been used for many years, the method usually gives magnetic powder having an average size in the range of 5-10 microns. If such the coarse powder is used for preparing a recording tape, the tape will have a rough surface which results in such disadvantages as a high noise level, difficulty in maintaining intimate contact of the recording surface with the magnetic head of tape recorder, and serious abrasion of the magnetic head. Thus, it is difficult to achieve a satisfactory high density recording with the coarse magnetic material.
The abovementioned method (3) involves a complicated operation and requires expensive apparatus and, thus, from the economic view point the commercial use thereof in a large scale is impractical.
In the abovementioned method (5) comprising electrolysis of a metal salt with a mercury cathode, the product is deposited on the cathode as particles in the form of dendrite containing about 4-6% of mercury. The dendrite particles are heated to remove the mercury. However, it is very difficult fo completely remove the entrained mercury from the product. Further, this process includes a danger of polluting the environment with the mercury. These difficulties are a barrier to the practice of the electrolysis method.
According to the abovementioned method (6), the reduction of a salt or salts in solution will produce a metal or alloy powder which has a highly reactive surface and, thus, is susceptible to oxidation in the presence of oxygen and moisture and eventually may give rise to spontaneous combustion. The powder tends to oxidize slowly even at room temperature and humidity conditions with the consequent deterioration of the desirable magnetic characteristics. The particles of powder obtained by the above method (6) are microscopic fibrils in which individual particles adhere mutually into line. One skilled in the art will appreciate that this type of structure is desirable for the magnetic material. However, when the product is used for the production of recording tape, this characteristic shape is often lost during the stage of admixing the material with a resinuous binder to form an uniform suspension. This results in the loss of a large part of the orientation property of the material, resulting in lowering of magnetic characteristics, particularly the square ratio, of the magnetic recording medium prepared therewith.
The present invention relates to improvement of the abovementioned method (2) generally comprising a step of reducing an iron oxyhydroxide or oxide with a reducing gas. Some disadvantages of the prior method (2) have been pointed out. Since in the prior method the reducing treatment is commonly carried out in a hydrogen stream at an elevated temperature, the starting material is subject to reduction in the volume due to the elimination of oxygen, change in the appearance to a porous texture, change in the microstructural shape, and sintering of particles into lumps. Therefore, even if the particles of starting material have a microstructural shape which is desired in the product magnetic powder, in the prior method, it is difficult to achieve desirable magnetic characteristics in the resulting product.
Similar deteriorative phenomena have been found in the conventional production of .gamma.-Fe.sub.2 O.sub.3, particularly in an intermediate step thereof in which .alpha.-Fe.sub.2 O.sub.3 is converted into Fe.sub.3 O.sub.4 with gaseous hydrogen by eliminating only 1/9 of oxygen content of the .alpha.-Fe.sub.2 O.sub.3. It is said that during said conversion step the material is subject to substantial change in the microstructural shape and sintering of particles.
When a metal is prepared from the oxide, it is expected that the deteriorative phenomena will occur to a much greater extent than in the above .gamma.-Fe.sub.2 O.sub.3 production, since substantially all the oxygen content must be removed from the oxide. Thus, the product powder prepared from the oxide will have a low coercive force (Hc) and square ratio, and will not provide a uniform dispersion in a resinous binder composition when used in tape production.
So far as we know the prior method (2) has not yet provided a magnetic powder having characteristics which give satisfactory results in practice. The product metal or alloy powder has a further practical drawback in that it is combustible.